1. Field of the Invention
The present invention relates to a switching device using S-N-S superlattice.
2. Description of the Related Art
Since the discovery of the transistor effect by William Bradford Shockley, John Bardeen and Walter H. Brattain in 1948, the semiconductor device for computing has generated a tremendous number of concepts related to the revolution of the world. The fastest speed of data processing is required to synchronize with the stream of consciousness. In the field of electronic devices, in order to catch up with such a stream, the circuit elements have been downsized to satisfy the “Scaling Rule” which suggests that the processing speed should be improved by downsizing the circuit assembly, including Field Effect Transistors (FET), to reduce the product of resistivity and capacitance of integrated circuits. However, it is becoming increasingly difficult to meet the contemporary demands of high speed processing by the Scaling Rule. Since the switching operation of the FET is originated from motions of carriers neighboring the gate contact by bias voltage, the switching frequency depends upon the mobility of the carriers. Recently Y. Nish predicted that the chip frequency would be restricted approximately up to 1.1×109 Hz until the year of 2010 (Y. Nish, Proceedings of International Symposium on Control of Single Particles and its Application (1996)). According to the prediction, the performance of a Central Processing Unit (CPU) should be a little higher than the common units. That is to say that processing inflated amount of information by large-sized software should be restricted by the limit of the mobility of carriers inside semiconductors.
On the other hand, a switching device using superconducting material with a tunneling insulating barrier, predicted by Brian David Josephson, has been known to be a high frequency switching device, which consumes extremely low energy (B. D. Josephson, Phys. Rev. Lett., 1(7)(1962)251). However the Josephson junction device has never been put to practical use because the switching operation beyond a frequency of 7×108 Hz is suffered from chaotic noise. Besides, there are three properties such as attenuation of signals transmitted across the tunneling barrier, delay of signals by parasitic capacitance and mechanical fragility against thermal stress. The oxide superconductor discovered by K. Alex Muller and J. Georg Bednorz (K. Alex Muller and J. Georg Bednorz, Zeitschrift fur Physik, B64(1986)189), which is able to operate at higher temperature, has been introduced to the switching device. In spite of a lot of trial, switching devices using an oxide superconductor have never been practically used because of their own property. In this paper, we propose that we can solve all problems by using a metal superconductor superlattice.
There are two reasons why the oxide superconductor has never been applied to practical Josephson devices. First, the phase change of the wave function should be fluctuated by the existence of incoherent interfaces such as the grain boundaries. Second, it is difficult to integrate the device because of the low transmittance rate of the wave function across such incoherent interfaces. Coherent length of the oxide superconductor is designed to be shortest (shorter than 0.1 nm) to raise the superconducting critical temperature Tc (H. Hayakawa and Y. Takagi, Oyo Butsuri (in Japanese), 58(5)(1989)766). Such short coherent length is realized by inserting ionic layers with high electric polarization as the partitions of the coherent region. Accordingly, the delay of wave function between conducting layers cannot be avoided (FIG. 1A). Besides, the delay should not be uniform in the region neighboring the incoherent interface (FIG. 1B). By such rack of uniformity, thermal noises are preference beyond the switching frequency of 104 Hz (L. Hao, J. C. MacFarlane, C. M. Pegrum, Supercond. Sci. Technol., 9(1996)678) and dynamical impedance of the interface is increased (K. K. Likharev and V. K. Semenov, JETP. Lett., 15(1972)3537), therefore, transmittance of the wave function is lowered in the high frequency region. This means that these problems cannot be essentially solved by partitioning the coherent region by using dielectric material.
Superconductivity is decided by coherence of wave function of the Cooper pair, and the coherence is realized by spin exchange correlation. This means we can solve the problem of partitioning the conducting region by controlling spin exchange correlation. Then we are going to try to understand the mechanism of the Giant Magneto-Resistance (GMR) as an example of artificial control of spin exchange correlation. It is reported that GMR is realized in the metal system in a mesoscopic scale and resistivity is lowered by 50% in the applied magnetic field (eg. M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich and J. Chazelas, Phys. Rev. Lett., 61(1988)2472). GMR is originated from the fact that transition of itinerant electrons between the ferro-magnetic layers is restricted when the wave function considering spin direction is opposite to the next layer (Kondo effect (Jun Kondo, “An abstract of metal electron theory” (in Japanese), Shokabo Press (1983))). The experimental result on the spin ordering in the system revealing GMR has already been reported (N. Hosono, S. Araki, K. Mibu and T. Shinjo, J. Phys. Soc. Jpn., 59(6)(1990)1925). According to this report, the half-ordered reflection, which is the proof of spin ordering in the scale of the superlattice spacing, can be observed in the experiment of the neutron diffraction.
It should be emphasized that the well-controlled magnetic domain can be realized in the mesoscopic system by control of the superlattice spacing. This suggests that transition of itinerant electrons can be controlled by tuning the spacing of strongly correlated layers without inserting any dielectric insulator. According to this suggestion, we can also design the superlattice using correlated materials such as superconductor without any dielectric materials.